Mat switch with cleaning oil film

ABSTRACT

An electric mat switch assembly comprised of a multi-layer structure including a plurality of spaced metal contact plates having confronting contact surfaces and thin spacer means between the plates for normally maintaining said surfaces in closely spaced-apart relationship while permitting flexible movement into electrical engagement upon the application of pressure to the multilayer structure is provided with a layer of nonconductive nonvolatile fluid on at least one of said confronting contact surfaces. The fluid layer is a lubricating oil that adheres to the contact surface and suspends insulating metal oxide particles from the surfaces of the plates to impart increased mobility thereto for permitting electrical contact between the contact plates when the surfaces are compressed into intimate engagement.

United States Patent [1 1 Molava [4 1 Aug. 26, 1975 MAT SWITCH WITHCLEANING OIL FILM Andrew P. Molava, Wethersfield, Conn.

[75] Inventor:

[73] Assignee: The Stanley Works, New Britain,

Conn.

[22] Filed: Oct. 29, 1973 [21] Appl. No.: 410,614

200/DIG. 44, 160, 153 C, 159 B, 85 R, 86 R, 86.5, 285, 295, 302, 242;340/272 [56] References Cited UNITED STATES PATENTS Monroe 200/86 RPrimary Examiner-Gerald P. Tolin Attorney, Agent, or FirmPrutzman,Hayes, Kalb & Chilton 5 7 ABSTRACT An electric mat switch assemblycomprised of a multilayer structure including a plurality of spacedmetal contact plates having confronting contact surfaces and thin spacermeans between the plates for normally maintaining said surfaces inclosely spaced-apart relationship while permitting flexible movementinto electrical engagement upon the application of pressure to themultilayer structure is provided with a layer of nonconductivenonvolatile fluid on at least one of said confronting contact surfaces.The fluid layer is a lubricating oil that adheres to the contact surfaceand suspends insulating metal oxide particles from the surfaces of theplates to impart increased mobility thereto for permitting electricalcontact between the contact plates when the surfaces are compressed intointimate engagement.

6 Claims, 3 Drawing Figures MAT SWITCH WITH CLEANING OIL FILM BACKGROUNDAND SUMMARY OF THE INVENTION The present invention relates to matswitches for controlling the operation of electrically actuated doorsand the like. More particularly it relates to a new mat switchconstruction having improved response characteristics to pressureapplied thereto by pedestrian traffic moving thereacross.

As is well known, mat switches of the type described generally aremulti-layer sandwich assemblies comprised of several flexible metalplates maintained in normally closely spaced-apart relationship by thinnonconductive separators. The separators are configured to permit theuppermost metal plate to be depressed and the plates squeezed togetherby the weight of pedestrian traffic moving over the top surface of themat. It is important for the proper operation of mats of this type thatthe pressure sensitivity thereof be maintained between certainpredetermined pressure levels. If the responsive pressure level is settoo high, grocery carts or small children would not actuate the door andaccidents could result. Alternatively, an excessively low pressureresponse level could result in unnecessary and unwanted operation of thedoor controlled by the mat switch.

The metal plates in the mat assembly are thin enough to be flexible andresilient and are constructed of suitable conductive materials such assheet aluminum and the like. Although the mat switch is conventionallysealed about the periphery thereof, a limited degree of oxidativecorrosion occurs on the planar contact surfaces of the plates and, inthe case of aluminum, insulative oxides that reduce the conductivitysensitivity of the contact plates tend to form thereby adverselyaffecting the contact pressure required to effect dependable operationof the electrically-actuated doors controlled by the electric matswitch. For example, a mat switch having a minimum pressure loadsensitivity of about pounds when initially constructed and installed maylose its sensitivity over a period of one to two years so that a load of50 pounds is required to effect its operation. As can be appreciated,this difference can be important particularly where the pedestriantraffic includes small children whose weight may not be sufficient toapply the requisite pressure to the mat. Additionally, the thin metaloxide layer tends to granulate and fall off the plates. These granularparticles tend to cause accelerated abrasive wear of the assembly andcan cake to render isolated areas fully inoperative.

Accordingly, it is an object of the present invention to provide a newand improved electric mat switch assembly having a stabilized and moreuniform sensitivity over prolonged periods of operation.

Another object of the present invention is to provide a mat switch ofthe character described that exhibits not only a stabilized pressuresensitivity but also is subject to reduced wear characteristics therebyfurther enhancing the operating life of the mat.

Still another object of the present invention is to provide a new andimproved mat switch assembly that reduces the formation of metal oxidesand prevents the compacting thereof that might otherwise reduce thesensitivity of the plate and reduce the wearability thereof. Included inthis object is the provision for the foregoing advantages in a low-costeconomical manner that lends itself readily to mass-productiontechniques.

A further object is to provide a mat switch having means for maintainingsolid oxidative products in a mobile condition so as to reduce theirtendency to affect the sensitivity of the switch assembly. Included inthe object is the provision for fluid capable of suspending dustparticles located between the contact surfaces of the switch withouthampering the desired electrical performance of those surfaces and infact improving such performance, such as by acting as a sparkdepressant.

Other objects will be in part obvious and in part pointed out in moredetail hereinafter.

These and related objects are accomplished in accordance with thepresent invention by providing an electric mat switch assembly comprisedof a multi-layer structure including a plurality of spaced metal contactplates having confronting contact surfaces, thin spacer means betweenthe plates for normally maintaining said surfaces in closelyspaced-apart relationship while permitting flexible movement intoelectrical engagement upon the application of pressure to themulti-layer structure and a layer of nonconductive nonvolatile fluid onat least one of said confronting contact surfaces. The fluid layeradheres to the contact surface and suspends metal oxide particlesbetween the plates to impart increased mobility thereto for permittingelectrical contact between the contact plates when the surfaces arecompressed into intimate engagement.

A better understanding of the objects, advantages, features, propertiesand relationships of the invention will be obtained from the followingdetailed description and accompanying drawing which set forth anillustrative embodiment and are indicative of the way in which theprinciples of the invention are employed.

A BRIEF DESCRIPTION OF THE DRAWING In the drawings:

FIG. 1 is a perspective view partially broken away and partially insection illustrating a mat switch assembly incorporating the features ofthe present invention;

FIG. 2 is an enlarged sectional view taken along the line 22 of FIG. 1with some of the layers of the multilayer structure exaggerated in sizefor illustrative purposes and;

FIG. 3 is a still further enlarged sectional view of a portion of a matswitch embodying the features of the present invention.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to the drawings ingreater detail wherein like reference numerals indicate like partsthroughout the several figures, mat switch 10 is shown as comprised of athree-conductor plate multi-layer assembly of the general type describedin U.S. Pat. No. 2,700,080 with a modified spacer construction. Thecontact plate assembly 12 of the switch 10 is enclosed in a suitableweather-proof envelope of rubber or the like consisting of a base layer14 and a top layer 16 that is provided with a corrugated top surface 18in a conventional manner to provide a wear-resistant tread for the matswitch. The contact plate assembly 12 is best shown in FIG. 2 asconsisting of a top plate 20 firmly adhered to the underside of theweather-proof tread layer 16 and a lower or bottom contact sheet 22resting on the base layer 14 and electrically connected to the top plate20 by means of a suitable rivet assembly or the like (not shown) and toa first common lead of'the electrical terminal 24. The intermediate ormiddle contact plate 26 is sandwiched between the upper and lowerplates, 20, 22 and is electrically connected to a secondlead of theterminal 24 so that contact between plate 26 and either plate 20, or 22will effect a closed circuit condition.

Adjacent plates of the contact assembly 12 are separated by pieces ofinsulating material preferably formed as integral thin sheets ofnon-conductive material provided with a plurality of apertures spaced atregular intervals to permit electrical contact between the intermediateswitching plate and either the top or bottom contact plates. In theparticular embodiment illustrated, the insulating material is shown astaking the form of a pair of insulating sheets on opposite sides of theswitching plate 26. Thus the spacer array between plates 26 and 20includes a pair of sheets having apertures that are in registry but areof different size to form a step-wise construction reducing the amountof flexing or bending required by the contact and switching plates inorder toprovide the desired closed circuit condition. Thus, as bestshown in FIG. 1, the uppermost sheet-like spacer, which may be of thinnon-conductive materialsuch as heavy paper or the like, is preferablycut to form a stencil having a plurality of rectangular strips 30extending across the sheet in spaced parallel relationship. The secondspacer in the array immediately adjacent the middle switching plate 26is of similar construction except that the rectangular strips 32 formedtherein are substantially narrower but are in registry with theoverlying broader strips 30 in order to provide the step-wiseconfiguration mentioned hereinbefore.

A similar spacer array is provided between the central contact plate 26and the lower contact plate 22 except that the lower spacers arearranged in 'mirror image fashion relative to the top spacers 30, 32 andare offset therefrom. Thus the lowermost spacer adjacent plate 22 isprovided with the broad rectangular strips 34 while the lower spacerclosest to the center contact plate 26 is provided with the narrowerrectangular strips 36. Additionally, the lower spacer array is laterallyspaced relative to the upper spacer assembly so that the rectangulartransversely extending strips 34, 36 are disposed intermediate therectangular strips 30, 32 in the upper array. As will be appreciated,this staggered or laterally displaced arrangement will facilitate a moregradual deflection under pressure in the upper or middle plates in orderto make the necessary electrical contact therebetween.

As mentioned hereinbefore, the metal contact plates are generallysubject to oxidative corrosion and in the case of the aluminum sheetmaterial used for the contact plates, an aluminum oxide is formed. Thealuminum oxide exhibits an electrically insulating character which tendsto interfere with the desired electrical contact between the contactplates. Additionally, the aluminum oxide is an abrasive material whichcuts away at the paper spacers and abrasively wears the contact platesthemselves. This abrasive insulating dust formed within theweather-proof rubber envelope can rapidly and seriously interfere withthe operation of the mat switch.

In accordance with the present invention, the formation of theundesirable oxide film is substantially reduced and the oxide which isformed is maintained in a flowable or'suspended condition so as not tocause abrasive wear or interfere with the desired contact between thecontact plates of the mat switch assembly. This is accomplished bycoating the contact plate with a thin coating of a viscous nonconductiveliquid capable of forming an adhering film on the surfaces of the platewhile suspending oxide particles therein and permitting intimateelectrically conductive surface contact between the contact plates whenthey are pressedinto engagement by pedestrian traffic applying pressureto the top tread surface 18 of the mat.

The liquid found most suitable for this purpose is a mineral oil readilyadapted to application by roller coating techniques. In the assembly 10used for illustrative purposes the mineral oil is applied to both sidesof the center contact plate 26 and is designated by the numerals 38,40(see FIG. 3). Of course, as will be appreciated, the oil is picked up bythe top and bottom plates during use of the mat-However, initially, itis only necessary that the coating be applied to the central plate 26 inorder to provide the desired. stabilized sensitivity characteristics. Inthis regard, electrical mat switches incorporating the coated oil on acontact plate thereof have shown no substantial increase in the minimumpressure value. required to affect contact, even after to six mils. Aswill be appreciated, the liquid should be substantially free of highlyvolatile components and should be relatively stable over widetemperature. conditions in view of the fact the electric mat switch maybe placed in a location subject to seasonal temperature changes or maybe placed in environments where the temperatures will varysubstantially. Among the privinary requirements of the oil are itsability to adhere readily to the surface to which it is applied as athin coating and to suspend the small granular metal oxides formed onthe surface of the contact plates. The suspended granules designated bythe numeral 42 can then more readily flow away from the area of contactbetween the adjacent plates as the plates are compressed intoengagement. This reduces the interference to electrical contact andproviding a more stabilized sensitivity in the mat over a prolonged worklife. Additionally, the oil should be substantially insulative incharacter and capable of readily flowing so as to permit engagementbetween the contact plates when pressure is applied to the top of themat.

Although a number of different high quality mineral oils for lubricatingmachinery bearings or the like. may

be employed, one specific oil that has been used with good results is aparaffinic straight mineral machine and engine oil sold by AtlanticRichfield Company under the trademark Rubilene 8-315. This material hasa viscosity of 310 seconds Saybolt at F and 53 seconds Saybolt at 2l0Fwith a viscosity index of 95. It has a carbon residue of only 0.05percent by weight and exhibits a flash temperature of 430F.

The oil not only functions to suspend the insulating granular orpowdered material formed on the contact plates but also acts as alubricant to resist wear between the contact plates and the insulatingspacers whereby further enhancing and prolonging the operating life of 5the carpet. The effectiveness of the oil is particularly surprisingsince many electrical systems employing contacts will not operateeffectively when the contact surfaces are coated with a nonconductiveoil. However, in the present invention the oil tends to disperse inconductive granules so as to stabilize the operation of the contactplates, remove oxides therefrom and act as a spark depressant. The oilprovides an environment which prevents buildup or caking of theinsulative aluminum oxides on the contacts and tends to insulate thecontact surface from the air sealed within the assembly' As will beappreciated by persons skilled in the art, various modifications,adaptations and variations of the foregoing specific disclosure can bemade without departing from the teachings of the present invention.

I claim:

1. An electric mat switch comprised of a multi-layer structure includinga plurality of spaced metal contact plates having confronting contactsurfaces subject to the formation during use of granular metal oxideparticles that can reduce the pressure sensitivity of the plates, thinspacer means between the plates for normally maintaining said surfacesin closely spaced-apart relationship while permitting flexible movementinto electrical engagement upon the application of pressure to themulti-layer structure, said flexible movement being sufficient to causesaid granular particles to become loosened from the contact surfaces,and a layer of nonconductive nonvolatile fluid on at least one of saidconfronting surfaces, said fluid layer being firmly adhered to thecontact surface and uniformly suspending said loosened granularparticles and imparting mobility thereto, said layer permittingelectrical contact between said contact plates when said surfaces aremoved into intimate engagement and maintaining a stabilized pressuresensitivity in the switch over a prolonged period of use.

2. The mat switch of claim 1 wherein the fluid is a medium viscosityliquid.

3. The mat switch of claim 1 wherein the fluid is a mineral oil.

4. The mat switch of claim 1 wherein the fluid layer has a filmthickness of about two to six mils.

5. The mat switch of claim 1 wherein the multilayer structure includesthree metal contact plates and the planar surfaces of the intermediatecontact plate each carry a layer of the nonconductive liquid.

6. The mat switch of claim 5 wherein the liquid is a paraffinic oilhaving a viscosity up to about 500 seconds Saybolt at F.

1. An electric mat switch comprised of a multi-layer structure includinga plurality of spaced metal contact plates having confronting contactsurfaces subject to the formation during use of granular metal oxideparticles that can reduce the pressure sensitivity of the plates, thinspacer means between the plates for normally maintaining said surfacesin closely spaced-apart relationship while permitting flexible movementinto electrical engagement upon the application of pressure to themulti-layer structure, said flexible movement being sufficient to causesaid granular particles to become loosened from the contact surfaces,and a layer of nonconductive nonvolatile fluid on at least one of saidconfronting surfaces, said fluid layer being firmly adhered to thecontact surface and uniformly suspending said loosened granularparticles and imparting mobility thereto, said layer permittingelectrical contact between said contact plates when said surfaces aremoved into intimate engagement and maintaining a stabilized pressuresensitivity in the switch over a prolonged period of use.
 2. The matswitch of claim 1 wherein the fluid is a medium viscosity liquid.
 3. Themat switch of claim 1 wherein the fluid is a mineral oil.
 4. The matswitch of claim 1 wherein the fluid layer has a film thickness of abouttwo to six mils.
 5. The mat switch of claim 1 wherein the multilayerstructure includes three metal contact plates and the planar surfaces ofthe intermediate contact plate each carry a layer of the nonconductiveliquid.
 6. The mat switch of claim 5 wherein the liquid is a paraffinicoil having a viscosity up to about 500 seconds Saybolt at 100*F.